Bob Crow, the general secretary of the Rail Maritime and Transport union (RMT), died from a heart attack, according to reports.
It has been confirmed Mr Crow, who lived in Woodford Green, died at Whipps Cross Hospital, Leytonstone, at around 7am this morning at the age of 52.
He is said to have suffered an aneurysm and a massive heart attack, while doctors spent an hour trying to save his life.
The Mayor of London, Boris Johnson, who often criticised the RMT over Tube strikes, has paid tribute to Mr Crow.
He said: “I'm shocked. Bob Crow was a fighter and a man of character. Whatever our political differences, and there were many, this is tragic news.
“Bob fought tirelessly for his beliefs and for his members.
“There can be absolutely no doubt that he played a big part in the success of the Tube, and he shared my goal to make transport in London an even greater success. It's a sad day.”
The RMT offices will be closed for the rest of the day and further announcements are expected.
The Guardian is awaiting further information from Whipps Cross Hospital.